Information on how continuous noise is measured in the UK
Continuous noise is usually measured as Leq, which means the 'equivalent continuous sound level'. This basically works out the average sound level for a specific location over a defined measurement period – for instance the aircraft noise over one village between 7.00am and 11.00pm.
Following several large-scale studies, the UK Government has decided that if Leq is on average over 57dBA all day long, it begins to be of significant annoyance to the local community. This doesn’t mean every individual will be annoyed by the noise – some will, some won’t, and some people in areas of lower Leq will also be annoyed – but it provides a practical basis to identify the places where noise levels are particularly high.
In the UK, Leq noise contours – like height contours on an Ordnance Survey map, but showing increasing noise levels – are produced for the average summer day, measured from 16 June to 15 September, between 7.00am and 11.00pm. Leq contours are normally plotted from 57 to 72dBA at 3dB intervals. Some airports also routinely produce average summer Lnight contours covering the eight hours between 11.00pm and 7.00am and normally plotted from 48 to 66dBA at 3dB steps.
There are two main reasons for creating noise contours. Firstly, it makes it possible to identify how many people live or work in areas where there is significant annoyance from noise. The Government has stated an aim to reduce the number of people who are affected by noise; by reassessing noise contours each year, it is possible to assess progress against this aim.
Secondly, it helps with planning decisions. Projected Leq can be calculated for proposals such as airport extension or construction, and so the likely noise impact identified, and planners can work out ways to reduce noise exposure in the more affected areas. Importantly though, even if there are lots of areas where projected Leq is over 57dBA, it doesn’t necessarily mean that the proposals will be rejected as planners must balance economic, environmental and social factors when taking their decision about proposals.
Alongside the UK’s Leq measure, within the EU a slightly different approach is used to measure continuous noise. This is Lden, which starts by measuring the annual average 24-hour Leq, then adds additional weighting of 5dB for noise made in the evening (7.00pm-11.00pm) and 10dB for night-time (11.00pm-7.00am). It is generally assumed that 55dBA Lden is the point at which significant annoyance occurs. There is no precise relationship between Lden and Leq, it depends on the relative number of operations in the day, evening and night periods, however for many UK airports Lden is approximately just under 2dB higher than Leq 16 hour, all other things being equal.
The annual noise contours for Heathrow, Gatwick and Stansted airports dating back to 1997 are available from the Department for Transport (DfT) website.